Recently, biofuels gain attention as an alternative fuel to petroleum. Thus, commercial production of bioethanol using biomass such as sugar cane or corn is carried out. However, when food sources are used as a raw material of bioethanol, a problem occurs in that the prices fluctuate greatly due to a competition with the food sources. For such reasons, it is desired to produce biofuels by using cellulose-based biomass as non-food sources including tree, grass, rice straw, or the like.
Meanwhile, it is not easy to hydrolyze tough cellulose into sugar. Although a method of saccharifying cellulose using a strong liquid acid such as sulfuric acid is conventionally known, there are problems that an apparatus is corroded by the strong acid, gypsum or the like is generated in a large amount as a waste when a strong acid is treated for neutralization, or the like, and thus the method has not been put into practical use.
To solve those problems, a hydrothermal treatment for converting cellulose into water soluble polysaccharides with a low molecular weight using pressurized hot water but without using any catalyst gains attention (for example, Patent Documents 1 and 2). For the hydrothermal treatment, “pressurized hot water” is used. The pressurized hot water indicates water present in a liquid state at a high temperature and high pressure according to pressurization to the saturated vapor pressure or higher. It is believed that, as ionic product is increased, the pressurized hot water can promote the hydrolysis reaction of cellulose (see, paragraph [0024] of Patent Document 1). Accordingly, the hydrothermal treatment has an advantage that a cellulose raw material can be solubilized in a short period of time without using a specific chemical, and thus it can be said that it is a method for solubilizing a cellulose raw material having little load on an environment.
A method for chemical decomposition of cellulose is disclosed in Patent Document 3.